The effects of dietary nickel exposure on growth and reproduction of Daphnia magna

Abstract

Although there is growing evidence that dietborne metals can be toxic to various aquatic species, there is still insufficient knowledge to integrate this information in environmental risk assessment procedures. In this study, we investigated the effects of a 21-day exposure of Daphnia magna to a control diet (i.e. the green alga Pseudokirchneriella subcapitata containing <4.0 μg Ni/g dry wt) and five diets with elevated Ni concentrations (i.e. the same alga contaminated with Ni burdens between 33.7 and 837 μg Ni/g dry wt). A significant accumulation of dietborne Ni in D. magna, i.e. between 49.6 and 72.5 μg Ni/g dry wt, was observed when they were fed with diets containing between 85.6 and 837 μg Ni/g dry wt. This was paralleled by a significant reduction of reproduction (by 33.1%), measured as the total number of juvenile offspring per female and growth (by 9.1%), measured as the carapax length of 21-day-old females. Life-history analysis showed that the time to first brood of Ni exposed organisms was between 7.8 and 8.2 days, and occurred 0.7–1.1 days earlier than for the control organisms (time to first brood = 8.9 days). The number of offspring in the first brood was significantly reduced (by 21–33% compared to the control) in all dietary treatments. Longer exposure (≥8.9 days, i.e. from the second brood onwards) led to a reduction of brood size only when given diets containing 85.6 and 837 μg Ni/g dry wt. The results suggest that a variety of mechanisms may be involved in the effects of dietary Ni exposure, including altered resource allocation or targeted reproductive inhibition. While Ni exposure clearly altered the quality of the diet (measured as essential ω3 polyunsaturated fatty acid content and C:P ratio), we found no conclusive evidence that these diet quality shifts could have affected growth or total reproductive output. More research is required to fully understand the mechanisms of Ni toxicity associated with the dietary exposure route.